# Re: \[PATCH net v6 0/5\] more virtio-net csum hardening

- From: Michael S. Tsirkin \<mst@redhat.com\>
- Date: 2026-10-24 15:42:28 
- List: virtualization
- Page: https://ratatoskr.run/virtualization/2026/10/17798455
- Thread: https://ratatoskr.run/netdev/2026/10/17794265/t.md

```text
On Fri, Oct 23, 2026 at 11:09:17AM -0400, Michael S. Tsirkin wrote:
> 
> Okay this seems to be converging.... But sashiko did find some issues I
> thought are best fixed. Specifically I thought of a better way to handle
> short gso in a tunnel - just clear tunnel gso! As a side effect we now
> fix up the checksum for these dodgy packets like we did before, instead
> of a drop. Good. And more false positives that prompted me to write a
> bit more comments. So here goes.
> 
> Apologies for reposting so frequently, I know netdev bot will be unhappy
> with that.  But Paulos's report made a lot of publicity, there is a lot
> of people looking into this, so I feel there's value in defending
> against all this class of issues sooner rather than later.
> 
> The issues were mostly found by code inspection, but all reproduced by
> me in a VM.

Ugh. I found another wrinkle( Firewalls that shape the payload.
They are currently seem to simply be hoping most traffic is CHECKSUM_PARTIAL and
only looking at that, and that will break.

I am not sure what to do yet. Extending the sysctl knob to control what
happens if validation fails: pass/drop/complete - is the best I could
come up with. Or ignore the issue. Or just forget the complete at input hack
and drop if we can't figure it out.

> ------
> 
> As reported first by Paulos Yibelo [ref] (after which
> others, credited below, found more ways the bug can affect users):
> 
> allowing untrusted hardware to specify csum start/offset for
> CHECKSUM_PARTIAL was not a good idea, because, if it edits the packet
> outside the actual checksum, and depending on the setup, the networking
> stack could be looking at the packet both before and after the edit, and
> seeing different results.
> 
> Previously, commit 6785011f8b16 ("ipv4: validate checksum_start before
> completing checksum") validated all known instances where this
> causes the most grief, but the real fix is to make sure
> that either only actual checksum fields are ever edited,
> or the checksum is completed before the net stack looks at it,
> so it is always consistent.
> 
> 
> This basically goes back to
> what we did before f43798c27684 ("tun: Allow GSO using virtio_net_hdr"),
> except in a narrow list of specified known good cases.
> 
> 
> Summary of the patchset:
> 
> These are probabably net material (and seem to be too big for stable,
> or I would recommend backporting there, too):
> 
>   net: account for ingress-resolved virtio net hdr state
> 	- accounting, not the fix itself, dependency of the fix
>   um: vector: trim skbs before verifying the header
> 	- fixes the um receive path: without it the checksum would be
> 	  completed over the unreceived tail of the buffer
>   virtio_net: parse tunnel csum_start little-endian
> 	- bugfix, a dependency of validating the header
>   net: resolve uncommon partial checksums at ingress
> 	- fix itself
>   net: add a sysctl to opt out of virtio net hdr checksum validation
> 	- reducing risk of the fix by adding a sysctl to disable it
> 
> These are cleanups, can be deferred to net-next, too:
> 
>   net: add ip4_parse()
> 
> Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr")
> 
> Changes since v1:
> 	- reorder patches to fix bisectability, addressing
> 	  a comment by Björn Töpel.
> 	- code refactoring, different patch splitup,
> 	  a sysctl knob to disable as suggested by Willem de Bruijn.
> 	- fix a bunch of minor issues spotted by LLMs
> 
> 
> Changes since v2:
> 
> virtio_net: parse tunnel csum_start little-endian
>   - new patch. correctly parsing the header is a dependency
>     of validating it, otherwise i do not know what to validate
> 
> net: account for ingress-resolved virtio net hdr state
>   - make rx_csum_sw counter 64 bit so it won't wrap
>     (spotted by LLM review)
> 
> net: resolve uncommon partial checksums at ingress
>   - tunnel fixes to bugs noticed by Willem de Bruijn and
>     Chris Arges
>   - restore the csum_offset checks as suggested by Willem
> 
> net: add a sysctl to opt out of virtio net hdr checksum validation
>   - the knob is no longer writable by root inside a user
>     namespace: changing it now requires capable(CAP_NET_ADMIN)
>     suggested by Willem de Bruijn
> 
> net: add ip4_parse()
>   - comment converted to kernel-doc format (Willem de Bruijn)
> 
> um: vector: trim skbs before verifying the header
>   - new patch, found by code review
> 
> 
> Changes since v3:
> 
> net: account for ingress-resolved virtio net hdr state
>   - um: vector: honor virtio net hdr conversion failures
>     folded in (Björn Töpel)
>   - count on tap and virtio_net, too (Willem de Bruijn)
> 
> net: resolve uncommon partial checksums at ingress
>   - walk the frame on loopback, too: loopback_xmit() runs
>     eth_type_trans() on it (Willem de Bruijn)
>   - return the mismatch handling directly, GSO is implied in
>     the tunnel path so drop there outright, and fold the helper
>     into its one remaining caller (Willem de Bruijn)
>   - read the IP version through struct iphdr (Willem de Bruijn)
>   - document the ipv6_find_hdr() flags choice
> 
> net: add a sysctl to opt out of virtio net hdr checksum validation
>   - renamed to virtio_net_hdr_validate, in case more validation
>     is gated by the same knob later (Willem de Bruijn)
>   - point .data at the value like the neighboring entries,
>     the per-netns registration rebases it, and the handler reads
>     the value directly - no ctl_table copy (Björn Töpel,
>     Willem de Bruijn)
>   - check the opt-out before parsing: with the knob off, packets
>     the parser cannot handle were still dropped (Willem de Bruijn)
>   - include virtio_net.h in net/core/dev.c so the static key
>     definition sees its declaration (build warning reported by
>     Jakub Kicinski)
> 
> net: add ip4_parse()
>   - add the missing kernel-doc Return: section
>     (Jakub Kicinski)
> 
> 
> Changes since v4:
> 
> net: account for ingress-resolved virtio net hdr state
>   - count on tun only: dropped the tap, macvtap, ipvtap and
>     virtio_net counters (Willem de Bruijn)
>   - the driver side branches are in their final form here now,
>     moved from "net: resolve uncommon partial checksums at
>     ingress" (Willem de Bruijn)
> 
> net: resolve uncommon partial checksums at ingress
>   - no longer touches the drivers (Willem de Bruijn)
>   - a packet too small to be GSO: ignore the tunnel GSO request,
>     same as plain GSO ones, and complete wild metadata in place
>     (sashiko)
>   - document the zero copy interaction (sashiko)
>   - honor a preset skb->protocol on ethernet: the stack uses it,
>     except on loopback which re-parses the frame (sashiko)
> 
> net: add a sysctl to opt out of virtio net hdr checksum validation
>   - document the netns lifetime requirement (sashiko)
> 
> net: add ip4_parse()
>   - pull the whole header, options included, so that the
>     transport header offset is within the linear data
>     (Willem de Bruijn)
> 
> 
> Changes since v5:
> 
> net: resolve uncommon partial checksums at ingress
>   - read both skb->protocol and the frame ethertype: do not
>     validate when they disagree, instead of preferring the
>     preset protocol; same check after the IP version inference
>     where there is no L2 header (Willem de Bruijn)
>   - pull the IPv4 options in the helper as introduced, with the
>     transport header offset computed before the pull
>     (Willem de Bruijn)
>   - fold the trivial protocol calculation cleanup, formerly a
>     separate patch, in (Willem de Bruijn)
> 
> net: add ip4_parse()
>   - the options pull moved to the helper introduction in
>     "net: resolve uncommon partial checksums at ingress"
> 
> Tested: full suite on the final tree, tunnel included.
> um part compiled only.
> 
> 
> Michael S. Tsirkin (5):
>   net: account for ingress-resolved virtio net hdr state
>   um: vector: trim skbs before verifying the header
>   virtio_net: parse tunnel csum_start little-endian
>   net: resolve uncommon partial checksums at ingress
>   net: add a sysctl to opt out of virtio net hdr checksum validation
> 
>  Documentation/admin-guide/sysctl/net.rst |  19 ++
>  arch/um/drivers/vector_kern.c            |   6 +-
>  arch/um/drivers/vector_transports.c      |   4 +-
>  drivers/net/tap.c                        |   4 +-
>  drivers/net/tun.c                        |  41 ++-
>  drivers/net/virtio_net.c                 |   2 +-
>  include/linux/virtio_net.h               | 319 ++++++++++++++++++++++-
>  include/net/netns/core.h                 |   1 +
>  net/core/dev.c                           |   7 +
>  net/core/net_namespace.c                 |   1 +
>  net/core/sysctl_net_core.c               |  50 ++++
>  net/packet/af_packet.c                   |   4 +-
>  12 files changed, 441 insertions(+), 17 deletions(-)
> 
> -- 
> MST
> 

```